Coupling circuits



L/IVE on NETWORK UNBALANCED LINE 0/? NETWORK 5 UNBALANCED D. K. GANNETT COUPLING CIRCUITS Filed Oct. 50. '19:?

FIG.

FIG. 2

2 i i 2a INVENTOR D. K. GA N/VE T T BV ATTORNEY BALANCED LINE 0/? NETWORK BALANCED LINE OF? NE T WORK May 7, 1940.

Patented May 7, 1940 UNITED STATES COUPLING CIRCUITS Danforth K. Gannett, Mountain Lakes, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application October 30, 1937, Serial No. 171,917

1 Claim.

This invention relates to coupling circuits and more specifically to circuits for connecting a balanced line or network to an unbalanced line or network.

In the interconnection of a coaxial cable, one element of which is customarily connected to ground or to some other fixed potential, and balanced cable pairs, the elements of which are balanced with respect to ground or to some other fixed potential, an appropriate coupling circuit is necessary. It is well known to use transformers for coupling such a balanced circuit to an unbalanced circuit. In television transmission, however, where it is frequently necessary to. transmit a Wide range of frequencies, as, for example, from a few cycles to several million cycles, transformers are not satisfactory due to the fact that at the present stage of the transformer art, it is impossibleto provide a transformer which will transmit efiiciently such a wide range of frequencies.

It is an object of this invention to provide a circuit for coupling balanced lines or networks to unbalanced lines or networks in which coupling transformer coils are not used.

It is a further object of this invention to provide circuits for coupling balanced lines or networks to unbalanced lines or networks in which the longitudinal noise voltages which may be present in the balanced linesare balanced out in the coupling circuits.

In accordance with the invention, a coupling circuit is provided for interconnecting a balanced line or network and an unbalanced line or network which makes use of a phase inverting device such as an electron discharge tube. The input circuit of this phase inverting device is connected between one of the terminals of the balanced line or network and ground, While the output circuit of this device is connected directly to the unbalanced line or network. The other half of the balanced line or network is connected, without passing through the phase inverting device, to the terminals of the unbalanced line or network.

In a modification, one-half of the balanced line I or network is connected to the input of the first stage of a two-stage amplifier and the other half of the balanced line or network is connected to the input of a single stage amplifier. The anode of the output tube in each amplifier is connected to the terminal of the unbalanced line or network which is not connected to ground or to a fixed potential. The total gain of the two tubes comprising the two-stage amplifier is adjusted to equal the gain of the single tube in the one-stage amplifier. In this arrangement longitudinal noise potentials present in the balanced line or network are balanced out and are not present in the unbalanced line or network.

The invention will be more readily understood by referring to the following description taken in connection with the accompanying drawing forming a part thereof in which:

Fig. 1 shows a circuit for connecting a balanced line or network to an unbalanced line or network; and

Fig. 2 discloses a circuit which is a modification of that shown in Fig. 1.

Referring more particularly to the drawing, Fig. 1 shows a circuit H! for connecting a balanced line or network I!) toan unbalanced line or network I l. Connected across the terminals l3 and M of the balanced line or network It are the high resistances l5 and It, the common terminal ll of which is connected to ground (or to a fixed potential with respect to ground). A phase inverting tube It comprising a cathode 19, an anode 20 and a control grid 2! has its cathodegrid circuit connected by means of the coupling condenser 22 to the resistance 15. The customary resistance 23 and biasing source of potential 24 are also included in the grid-cathode circuit. The circuit between the cathode It and the anode'lll includes an anode resistance 25 and a source of anode potential 25. A tap 21 on the resistance 25 is connected by means of a coupling condenser 28 to the terminal 29 of the unbalanced line or network H, the other terminal 30 of which is connected to ground or to the potential of the common terminal I? of the resistances l5 and I6 if this potential is other than ground. By means of the tap 2! the gain of the electron discharge device I8 is adjusted so that the potential impressed on the point 32 from the electron discharge device is equal to the potential of the terminal l4 minus the drop of potential across the condenser 3!. The terminal M of the balanced line or network is connected through coupling condenser 3i, which is similar to the coupling condenser 22, to the point 32.

The ratio of the potential between terminals 29 and 30 of the unbalanced line or network to the current in terminal E3 of the balanced line or network can be shown by ordinary circuit theory to be,

Similarly, the ratio of the potential between the same points to the current in terminal I4 of the balanced network is Yo==admittance of the balanced network to ground from [3 or l4.

Z=impedance of the unbalanced network.

g1=admittance of the resistance [5.

g2=admittance of the resistance l6.

3=admittance of the resistance 23.

r =internal resistance of the electronic de-' vice l8. r1=resistance of the upper portion of the resistance 25. 2=resistance of the lower portion of the resistance 25.

This condition can be approximately realized, but since Y0 and Z are in general vectors with angles other than zero and amplitudes which vary with frequency while all the other quantities are constants of zero angle, it can be only approximate.

The signal which is to be transmitted may be represented by currents of is and +23 at terminals l3 and M, respectively, so that the signal potential across the unbalanced line or network is es=is(Z2-Z1).

Since due to the phase reversal Z1 is negativ it is evident that the two components of es are additive.

In the modification shown in Fig. 2, a stage of amplification is added to each side of the circuit shown in Fig. 1 so that a two-stage amplifier comprising the tubes [8 and 40 in tandem is connected across the resistance l5 and a singlestage amplifier comprising the tube 44 is connected across the resistance IS. The output circuits of the tubes 40 and 44 are connected in parallel.

The electron discharge device 40, comprising a cathode 4|, an anode 42 and a grid 43, has its grid-cathode circuit connected to the output circuit of the tube l8, while the tube 44, comprising a cathode 45, an anode 46, and a grid 41, has its grid-cathode circuit connected across the resistance iii. The resistance 48 and the biasing battery 49 complete the input circuit of the electron discharge device 40 while the resistance 50 and the biasing source of potential 5| complete the input circuit of the tube 44. The tubes 40 and 44 have a common output resistance 52 and source of anode potential 53, the two anodes 42 and 43 being connected to the terminal 29 of the unbalanced line or network II by means of the coupling condenser 54.

In operation, the tube it can be adjusted to have a zero gain and the tubes 40 and 44 to have any appropriate gain but equal to each other. As a modification to this arrangement, the gain of the tube H3 is not made equal to zero, but is of any desired value. In this arrangement, how- In the circuit shown in Fig. 2, the longitudinal noise currents through the resistance l5 are twice 4 reversed in phase so that they are approximately equal to the phase in the resistance l5, while the longitudinal noise currents through the resistance l6 are once reversed in phase by means of the phase inverting tube 44. Thus the longitudinal noise currents through resistances l5 and I6 are present in the resistance 52 but are substantiallydegrees out of phase with respect to each other. They therefore combine and balance out so that there are no noise components in the connection '55 leading to the terminal 29 resulting from the longitudinal noise in the balanced line or network. The signal currents, however, being in opposite phase at terminals 13 and I4 of the balanced line or network, are in the same phase in resistance 52 and therefore reinforce each other. The circuit of Fig. 2 has the advantage over that of Fig. 1 that the longitudinal noise currents may be accurately, instead of only approximately, balanced out. That this is true is evident without resort to equations from the symmetry of the impedances to ground at the input of the circuit of Fig. 2, and also at its output.

Various modifications may be made in the circuit described above without departing from the spirit of the invention, the scope of which is defined by the appended claim.

What is claimed is:

In a high frequency transmission system, a first circuit terminating in a shunt impedance the effective central point of which is maintained at a fixed potential and which has two external terminals, a second circuit, one terminal of Which is maintained at said fixed potential and the second terminal of which is free to vary in potential, and means coupling said circuits in energy transfer relation, said coupling means comprising an electron discharge device having an anode, a cathode and a control electrode, means for connecting the control electrode of said electron discharge device to one external terminal of said first circuit, means for maintaining the cathode of said device at said fixed potential, a second electron discharge device having an anode, a cathode and a control electrode, means for maintaining the cathode of said second electron discharge device at said fixed potential, means for connecting the control electrode of said second electron discharge device to the anode of the first electron discharge device, a third electron discharge device having an anode, a cathode and a control electrode, means for connecting the control electrode of said third electron discharge device to the other external terminal of said first circuit, means for maintaining the cathode of said third electron discharge device at said fixed potential, means for connecting the anode of said second and third electron discharge devices to the terminal of said second circuit which is free to vary, the gain of the third electron discharge device being equal to the total gain of the first and second electron discharge devices, and adjustable means in circuit with said discharge devices adapted to compensate for any inequality in said gains. 

